1. Field of the Invention
The present invention relates to high-frequency switches and arrays of high-frequency switches for high frequency signals with micromechanical switch elements, and a method of production of high-frequency switches using integrated circuit fabrication processes.
The present invention is applied to various systems including tuning circuits and transmission/receiving switches of wireless local-area-network systems, phased array antennas, matching-impedance converters, phase shifters, and high-frequency wave sources, which utilize high frequency signals at frequencies of millimeter and sub-millimeter waves.
2. Description of the Related Art
Recently, much attention is focused on high frequency signals at frequencies of millimeter and sub-millimeter waves, as means for high-speed transmission of a large amount of information, which can provide a wide range of available frequency bands for wireless communications. Performance of semiconductor elements at such frequencies of high frequency signals is significantly lowered. A microwave switch of a type having a field-effect transistor on a strip line shows a too small change between the ON-state impedance and the OFF-state impedance in response to a change of a control voltage, and it is difficult to achieve stable, high-speed switching ON/OFF actions.
U.S. Pat. No. 5,619,061 discloses a microwave switch having a micromechanical metal-coated cantilever which acts as a metal-to-metal switch. The microwave switch is fabricated using micromachining.
In the microwave switch of the above publication, a silicon-dioxide cantilever extends out over an opening etched in a silicon substrate. Metal electrodes extend onto the cantilever, and a metal conductor extends onto and up and out over the end of the cantilever. A metal contact on the silicon dioxide lies in the same plane as the cantilever and extends out under the end of the metal conductor.
The microwave switch of the above publication operates as follows. With no voltage applied between the electrodes and the substrate, the cantilever remains parallel to the surface of the substrate, and the switch is open. When a predetermined voltage is applied between the electrodes and the substrate, the cantilever is pulled toward the substrate until the end of the conductor makes contact with the metal contact. This closes the switch. Release of the pull-down voltage then opens the switch. The switch of the above publication is able to show a large change between the ON-state impedance and the OFF-state impedance in response to a change of the voltage, and it can achieve high-speed switching ON/OFF actions in response to the voltage.
Hereinafter, the cantilever or a switch element of this type that acts as the metal-to-metal switch will be called a deflectable air-bridge.
FIG. 4A shows an equivalent circuit of a conventional microwave switch array of the above publication. FIG. 4B shows an equivalent circuit of a phased array antenna utilizing the conventional microwave switch array of FIG. 4A. The conventional microwave switch array of the above publication will be described later, for the purpose of comparison between the conventional microwave switch array and the present invention.
However, there is a problem in the switch of the above publication. That is, the actuation voltage that actuates the switch-ON action of the cantilever depends on mechanical coefficients of the silicon oxide film of the cantilever, and a configuration of the cantilever (for example, the length, the width and the thickness) is determined by taking account of the electrical characteristics (for example, the ON-state impedance). Hence, the switch of the above publication inherently has a fixed value of the actuation voltage, and it is difficult to adjust the actuation voltage to match a particular actuation voltage selected for the switch. When an array of microwave switches of the above publication is formed on a substrate of an integrated circuit chip, each of the microwave switches has the fixed value of the actuation voltage. In order to control individual switching ON/OFF actions of the microwave switches, it is necessary to provide a corresponding number of control-signal conductors, which supply the fixed actuation voltages to the respective switches, on the substrate of the integrated circuit chip. The control-signal conductors provided on the substrate requires a comparatively large area of the integrated circuit chip, and it is difficult to increase an effective area for other elements of the integrated circuit chip on which the switch array is provided.
In practical applications, there is an increasing demand for a high-frequency switch which is operable to connect and disconnect a number of transmission lines through a number of contacts. U.S. Pat. No. 5,121,089 discloses a micromachined electrostatically actuated switch which is adapted to perform switching ON/OFF actions for a number of transmission lines via a number of contacts, and meets the demand. The switch of this publication is fabricated using integrated circuit fabrication processes.
However, the switch of the above publication includes a rotating switch blade which rotates about a hub formed on the substrate under the influence of electrostatic fields created by an actuation voltage. The switching ON/OFF actions are performed by the rotation of the switch blade, and it is difficult to achieve high-speed switching ON/OFF actions in response to the voltage. As the switch of the above publication has a relatively large delay of the switching ON/OFF actions, it is not suitable for use in a phased array antenna which requires high-speed phase shifting actions with the least possible delays.